Short Cycling, already combined Y1 and G wire

[Chef_E]

Just bought and installed Nest Learning Thermostat.

Fan was cycling on and off, then run a short time then cycle again. Someone on here said to jump the Y1 and G wires together. It's helped, there is no on/off cycling, however it's taken 2 hours to come down 3 degrees. AC will run for 5 minutes, off for 2-3, then on again.

Old system had 5 wires, one tied to a spot labeled "not used" is this the "C" wire?

I don't want to tax my AC unit, just want normal functionality and reliable cooling

[Houptee]

You are not supposed to jump any wires in the Nest.

G is the Fan control, so you can run the fan alone if the AC is off

Y is the control for the AC system, the outdoor unit to power on when a signal is sent to Y

If you jumper them together the fan cannot be controlled properly.

Do you have the Nest HVAC system type set up correctly for the type of system you have?

Is it a AC only system or is there Heating and Cooling controlled by the Nest?

[Chef_E]

Both the AC and furnace controlled by the thermostat. When I had the Y and G wires separate, the furnace fan would cycle on and off rapidly, then kick on for 5 minutes with the AC and then off again. I was afraid of damaging the system.

[Houptee]

It sounds like possibly a wire is loose, or the furnace control board might be faulty.

Go to the furnace and check the thermostat wires where they connect to the terminal strip are tight.

You can try removing the G wire at the Nest and run the AC see if the fan operates without the G wire. Some furnace control boards are designed to turn on the fan along with the AC and maybe this is the problem on your system.

[Chef_E]

Wires on furnace are tight. Removed the G wire at the nest, and used just the Y wire. AC runs constantly (according to nest it ran for 17+ hours yesterday) but furnace fan only runs for a minute or two before shutting off. Restarting 3-4 minutes later. That is inefficient cooling for sure, especially with a set point of 68 degrees. Nest says it'll take 2 hours to reach that temp from the current 71 degrees.

Attached is the furnace wiring. 

I'm going to install the old thermostat so I don't burn another 17 hours of electricity running an AC unit that isn't actually cooling my house without the fan.

 

[Houptee]

That was going to be my next suggestion is try the old Tstat to see if the system works properly.

The furnace main boards do go bad so its a god starting point to go back to old t stat.

If it still acts strange, depending on the brand and model furnace, the main board most likely has an updated part number. So if it needs a board get the newest version for your furnace.

[Chef_E · Accepted solution]

So it turns out my old system didn't utilize the "C" wire because it had a battery backup. When looking at the Tstat and the Nest, realized that I could use the "C" wire. Hooked it in, put the Y and G wires back and it's running great. No short cycles, we'll controlled heating and cooling. So happy.

[Jeran]

Hiya Chef_E,

I'm glad to see you were able to resolve this issue, and I appreciate you coming back with your solution! In that case, did you have any other questions or concerns?

Thank you for helping, Houptee!

Best regards,

Jeran

[Jeran]

Hey Chef_E,

Did you have any more questions or need any additional help? If not, I'll go ahead and lock up this thread in 24 hours.

Just checking up,
Jeran
 

[Chef_E]

nope, my Nest is running great!  I love its functionality and automation.

[Jeran]

Hiya Chief_E,

Awesome, I am glad to hear that! In that case, I'll go ahead and lock the thread. If you run into any more issues in the future, please feel free to create a new thread.

Have a good weekend!

Best regards,

Jeran